This invention relates to a process and a device for regulating the stimulation frequency of heart pacemakers. The device can thus be used in all patients requiring a heart pacemaker.
The invention has the aim, in heart pacemaker patients, of optimally covering the oxygen demand of the body via the blood circulation, so that the pulse frequency of the heart, as in the natural case, adapts to the particular load conditions, the optimum hemodynamic situation being found by the pacemaker itself.
In spite of the extended pacemaker function, the invention has, at the same time, the aim of virtually not changing the hitherto well-proved embodiment of the heart pacemakers and the associated pacemaker catheters, so that the known implantation techniques remain the same and, also, the high requirements with respect to the long-term use can be fulfilled.
In the case of heart pacemakers, controlling the pacemaker frequency via a measurement of the central venous oxygen saturation and thus adapting the frequency to the particular load conditions has already been described in DE-OS (German Offenlegungsschrift) No. 27 17 659. In this known process, the measurement of the blood oxygen saturation is carried out with the aid of a light-guide probe which is incorporated in the stimulation catheter. The measurement principle of reflection oximetry, which is used in the process, is based on the determination of the reflection intensities of light of the measured wavelength of 660 nm (R660) and of the reference wavelength of approximately 800 nm (R800) in the blood. A characteristic of the applied frequency adaptation is a firm association of the pacemaker frequency f with each determined measured value of the oxygen saturation S.sub.O2 EQU f=k.multidot.S.sub.O2, wherein f.sub.min &lt;f&lt;f.sub.max.
The values for k, f.sub.min and f.sub.max must be fixed before the implantation.
Furthermore, it is known from DE-OS (German Offenlegungsschrift) No. 21 13 247 that the ratio of the two measured values R.sub.800 /R.sub.660 is directly proportional to the blood oxygen saturation S.sub.O2 expressed by the ratio HbO2/(HbO2+Hb) (wherein Hb represents hemoglobin and HbO2 represents oxyhemoglobin).
The heart pacemakers constructed according to the principle indicated in DE-OS (German Offenlegungsschrift) No. 27 17 659 exhibit a number of problems which have hitherto hindered a clinical use of pacemakers of this kind:
The measurement method does not tolerate any changes in the optical transmission path (light guide, reflection space and coupling points), which cause a wavelength-dependent effect on the signal. This can occur through defective coupling points; material changes in the light guide; deposits on the light opening in the catheter, and foreign objects in the reflection region (heart wall, trabeculae).
Furthermore, the control of the pacemaker frequency in dependence on the oxygen saturation, according to a fixed prescribed characteristic curve, can have disadvantageous consequences if, on advancement of the basic cardiac illness, a change in the relationship of power of the heart to pulse frequency occurs. A deterioration in the hemodynamics can even occur through too great an increase in the frequency.
The measurement principle and control principle require a calibration before the implantation and correspondingly increase the service requirements.
In addition, the catheter has only a limited serviceability, since the end supports of the light guides become unstable over long periods because they have to accept the greatest part of the tensile strain acting on the catheter.
In addition, the fatigue strength of the light guides for long-term use is not given with the materials available at present, and the necessity of arranging the light aperture laterally in the combination catheter allows no margin technically for the further introduction of a mandrin (steel wire which is pushed into the highly elastic catheter during the implantation, in order more easily to introduce the catheter into the ventricle) beyond this point. In addition, the coupling system between the combination catheter and the pacemaker is much more complicated in the production, sensitive in use and voluminous than in the case of conventional pacemaker technology.
Since the energy losses while using the light-guide technique and the measuring principle mentioned can only be kept small by means of high optical precision, the costs for the total system compared with conventional technology increase several-fold.